[time-nuts] Trimble Resolution T - PPS offset and stability

Lachlan Gunn lachlan at twopif.net
Mon Sep 2 23:12:37 UTC 2013


It sounds like an experiment is in order, then.

To give some background, around six months ago I posted on here about the
idea of running an amateur common-view GPS network over the internet.  This
is all well and  good, but if I'm to have anyone else to compare with then I
will need to bring the hardware down to $20 from Farnell/Digikey/etc. and
five minutes of work.

At least initially, this will mean a microcontroller rather than an FPGA to
make the comparison.  I've been using a $15 STM32F4 devboard as a DSP
platform, but it has timers that can run at 168MHz, and can use them for
event timestamping.  Not as fast as I've done with FPGAs, but it could
certainly be worse.  Using something like this means that the timebase
stability will be awful, and so I'd rather use the PPS offset to avoid
degradation as a result if possible.

Thanks,
Lachlan

-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of Bob Camp
Sent: Tuesday, 3 September 2013 7:54 AM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Trimble Resolution T - PPS offset and stability

Hi

On all the GPS's I have tried it on, shifting the PPS has no real impact on
stability. A few things to consider:

Normally the shift is a few hundred ns either way The shift process is
always in steps of the main clock (100 ns for 10 MHz) GPS by it's self
bounces around a bit.

If you are talking about a shift of a big fraction of a second (and it
sounds like you are) then the stability of the GPS's local clock could come
into play. On something like a TBolt that's not going to matter. On a TCXO
based gizmo that is only corrected to 1.0x10^-7 you could get an extra 50 ns
of error at a half second offset. Weather you see that on this or that GPS
depends a lot on who wrote the firmware and what they worried about when
they did. 

The better alternative is to use a counter with a reasonable time base to
look at the difference between pps signals. If the counter has an OCXO time
base and it's properly calibrated you are about 10 to 100X better off than
the 50 ns in the example above. 

Bob

On Sep 2, 2013, at 6:05 PM, Lachlan Gunn <lachlan at twopif.net> wrote:

> Hello.
> 
> 
> 
> Has anyone here tried varying the PPS offset on a ResT (or UT+ or any 
> other GPS receiver for that matter) and measuring the resulting stability?
> 
> 
> 
> I ask because my Rb has only a 1PPS output, and while I have been able 
> to get at one of its internal HF signals, would like to see what I can 
> do with just 1PPS.  The obvious problem with doing this is that I will 
> need to align the PPS outputs together to get reasonable accuracy, but 
> I worry that a large offset in the GPS receiver will degrade stability 
> as the pulse moves away from the relevant packet in the GPS signal.
> 
> 
> 
> Am I being over-cautious?
> 
> 
> 
> Thanks,
> 
> Lachlan
> 
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